Technical Field
This disclosure relates generally to noise cancellation for signal sources (such as timing generators and local oscillators).
Related Art
A key specification of signal sources, such as local oscillators and timing generators based on phase locked loop (PLL) frequency synthesizers, is phase noise or period jitter. Among other things, the amount of phase noise or period jitter affects spectral usage efficiency and the ability to encode and decode high-speed data with minimal errors.
U.S. Pat. No. 8,750,441 describes a noise cancellation method to reduce phase noise, deterministic spectral components (some known as spurious components, or “spurs”), and other forms of angle noise, that can be present in the outputs of signal sources that are co-located or otherwise. This noise cancellation method includes: (a) canceling angle noise separately from the generating source itself, without the need to provide long term frequency stability and accuracy, and (b) canceling noise spectral components that are both inside and outside the loop bandwidth of phase locked loops that can be present within the signal source.
The amount of amplitude noise, such as noise added in the transmission path, can also be a key specification. In particular, the existence of unwanted amplitude noise can adversely affect other processes, such as frequency modulation and demodulation. One solution is to remove amplitude modulation from the carrier as received prior to delivering it to the angle modulation detector, such as using a limiter. Limiter circuits, however, consume significant power.